The Xbox One comes in three main flavors:
-The original Xbox One:
This is the big, black one that was available when they originally launched a few years back. They made 500GB and 1TB versions. They don't sell them new anymore, but used/refurbished ones are available at places like Gamestop. I'd probably avoid this one. Nothing really wrong with them (this is the one I have) but the other current choices are better.
-Xbox One S:
This is the "cheap" model that's in stores now (right around $200 to start), but by no means is it bad! It's slightly smaller than the original, and comes in 500GB and 1TB flavors. This one is slightly more powerful, and can stream 4K and play 4K Blu-Ray discs. It also has a HDR mode. The games still output 1080P, though.
-Xbox One X:
This is the BMW M5 of Xbox consoles, and currently the most powerful home console available. It's also the most expensive, at $400+. This one plays games in 4K with HDR, and even enhances some older games.
All of the Xbox One consoles play the same games. They fall into 3 categories:
-Xbox Originals:
These are the ones that were on the original "OG" Xbox that came out in 2001. Some, but not all, are backwards compatible and playable on the Xbox One.
-Xbox 360:
These were originally released on the older Xbox 360. Again, some, but not all, are backwards compatible and playable on the Xbox One. Most of the popular titles are playable, and they add new ones every month.
-Xbox One: These are the current ones available. The boxes are slightly smaller, like a Blu-Ray disc.
If you aren't used to modern gaming, the Xbox One does throw a slight learning curve. One thing is that you can't just pop in a disc and just play. The games need to partially install themselves on the system's hard drive, and discs basically act as a "key" to play the game when inserted. Basically everything available on physical discs is available online to download as well. I found out real early that you are going to want to pick up an additional external hard drive to store games on, especially if you get a 500GB model. Luckily, most USB hard drives work, so getting one should be easy.
Overall, it's a great console. It's a lot more reliable than the 360, and there are thousands of games for it and lots of backwards compatibility. Works fantastic as a streaming device, too!